Ms May formally notified the European Union of Britain's intention to leave the EU by triggering Article 50 of the treaty on March 29, setting the clock ticking on a two-year exit process, which has so far failed to yield a divorce deal.
"While the divorce talks proceed, the parties are still married. Reconciliation is still possible," John Kerr, British ambassador to the EU from 1990 to 1995, will say in a speech in London.
"We still have all the rights of a member-state, including the right to change our minds," Mr Kerr will say, according to excerpts released by his office. "The British people have the right to know this - they should not be misled."
The day Ms May triggered Article 50, she told the British parliament that there was "no turning back" and that the United Kingdom would be leaving the EU.
"A political decision has been made, in this country, to maintain that there can be no going back. Actually, the country still has a free choice about whether to proceed," Mr Kerr said.
In the shock referendum in June 2016, 17.4 million voters, or 51.9 percent of votes cast, backed leaving the EU while 16.1 million voters, or 48.1 percent of votes cast, backed staying.
Ms May, an initial opponent of Brexit who won the top job in the political turmoil that followed the vote, said last month that Britain would not revoke Article 50. Government lawyers asked judges last year to assume that it was irrevocable.
But ever since the referendum, opponents of Britain's exit - from French President Emmanuel Macron and former British Prime Minister Tony Blair to billionaire investor George Soros - have suggested Britain could change its mind.
European Council President Donald Tusk even invoked the lyrics of John Lennon to imagine a Brexit rescinded.
Thus far, there is no sign of a change of heart on Brexit in opinion polls. Both Ms May's Conservatives and the opposition Labour Party now explicitly support leaving the EU, which Britain joined in 1973.
Supporters of Brexit have repeatedly said that any attempt to have another referendum, or to undermine Brexit, would catapult the world's fifth largest economy into crisis.
"A second referendum would lead the United Kingdom into totally uncharted territory with very serious potential consequences for our democracy," said Richard Tice, who helped found one of the two Leave campaign groups in the referendum.
But the Brexit process has been challenged in a number of cases in British courts, many focusing on the as-yet unanswered question: Can Article 50 be reversed?
The 256-word clause does not say whether it can be revoked once it is invoked. This means that, if lawyers ask for clarification, the question would have to go to the European Court of Justice, the EU's highest court.
Mr Kerr, who in 2002-2003 acted as secretary-general of the European Constitutional Convention that drafted Article 50, said the debate had been misrepresented inside Britain: it was clear, he said, that Ms May's Article 50 letter could be revoked.
Such is the interest in the legalities of Brexit that one prominent lawyer, Jessica Simor, has formally asked for Ms May's unpublished legal advice on the matter.
"Britain can basically change its mind at any time right up to the 29th of March 2019," Mr Simor told Reuters last month.
"If you can revoke Article 50, then parliament has the power to rescue the country if that becomes necessary - if the government fails to secure a deal, or the deal is terrible, or the people do not want it."
(Writing by Guy Faulconbridge; Editing by Kevin Liffey)
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